Network Working Group M. Nottingham
Internet-Draft March 14, 2008
Expires: September 15, 2008
HTTP Header Linkingdraft-nottingham-http-link-header-01
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Copyright (C) The IETF Trust (2008).
Abstract
This document clarifies the status of the Link HTTP header and
attempts to consolidate link relations in a single registry.
Nottingham Expires September 15, 2008 [Page 1]

Internet-Draft HTTP Header Linking March 20081. Introduction
A means of indicating the relationships between documents on the Web
has been available for some time in HTML, and was considered as a
HTTP header in [RFC2068], but removed from [RFC2616], due to a lack
of implementation experience.
There have since surfaced many cases where a means of including this
information in HTTP headers has proved useful. However, because it
was removed, the status of the Link header is unclear, leading some
to consider minting new application-specific HTTP headers instead of
reusing it.
This document seeks to address these shortcomings.
Additionally, formats other than HTML -- namely, Atom [RFC4287] --
have also defined generic linking mechanisms that are similar to
those in HTML, but not identical. This document aims to reconcile
these differences when such links are expressed as headers.
This document does not attempt to specify how an XLink
[W3C.REC-xlink-20010627] might be mapped into the Link header, but
does not prohibit this from being done.
[[ NOTE: This is a straw-man draft that is intended to give a ROUGH
idea of what it would take to align and consolidate the HTML and Atom
link relations into a single registry with reasonable extensibility
rules. In particular; a) it changes the registry for Atom link
relations, and the process for registration; b) it assigns more
generic semantics to several existing link relations, both Atom and
HTML; c) it changes the syntax of the Link header (in the case where
extensions are present). Feedback is welcome on the
ietf-http-wg@w3.org mailing list, although this is NOT a work item of
the HTTPBIS WG. ]]
2. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, [RFC2119], as
scoped to those conformance targets.
This document uses the Augmented Backus-Naur Form (ABNF) notation of
[RFC2616], and explicitly includes the following rules from it:
quoted-string, token, SP (space). Additionally, the following rules
are included from [RFC3986]: URI-Reference.
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Internet-Draft HTTP Header Linking March 20083. The Link Header Field
The Link entity-header field provides a means for describing a
relationship between two resources, generally between that of the
entity associated with the header and some other resource. An entity
MAY include multiple Link values.
The Link header field is semantically equivalent to the <LINK>
element in HTML, as well as the atom:link element in Atom [RFC4287].
Link = "Link" ":" #("<" URI-Reference ">"
*( ";" link-param ) )
link-param = ( ( "rel" "=" relationship )
| ( "rev" "=" relationship )
| ( "title" "=" quoted-string )
| ( link-extension ) )
link-extension = token [ "=" ( token | quoted-string ) ]
relationship = URI-Reference |
<"> URI-Reference *( SP URI-Reference) <"> )
The title parameter MAY be used to label the destination of a link
such that it can be used as identification within a human-readable
menu.
Examples of usage include:
Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"
Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"
The first example indicates that chapter2 is previous to this
resource in a logical navigation path. The second indicates that the
person responsible for making the resource available is identified by
the given e-mail address.
3.1. Link Relationships
Relationship values are URIs that identify the type of link. If the
relationship is a relative URI, its base URI MUST be considered to be
"http://www.iana.org/assignments/link-relations.html#", and the value
MUST be present in the link relation registry.
4. IANA ConsiderationsNottingham Expires September 15, 2008 [Page 4]

Internet-Draft HTTP Header Linking March 20084.1. Link Header Registration
This specification requires that the Message Header Registry entry
for "Link" in HTTP [RFC3864] be updated to refer to this document.
Header field: Link
Applicable protocol: http
Status: standard
Author/change controller:
IETF (iesg@ietf.org)
Internet Engineering Task Force
Specification document(s):
[ this document ]
4.2. Link Relation Registry
This specification is intended to update Atom to become the reference
for the Link Relation registry, and clarifies its nature and use.
A Link relation is a way of indicating the semantics of a link. Link
relations are not format-specific, and MUST NOT specify a particular
format or media type that they are to be used with.
The security considerations of following a particular link are not
determined by the link's relation type; they are determined by the
specific context of the use and the media type of the response.
Likewise, a link relation SHOULD NOT specify what the context of its
use is, although the media type of the dereferenced link may
constrain how it is applied.
New relations MAY be registered, subject to IESG Approval, as
outlined in [RFC2434]. Requests should be made by email to IANA,
which will then forward the request to the IESG, requesting approval.
The request should use the following template:
o Relation Name:
o Description:
o Reference:
The Link Relation registry replaces the Atom Link Relation registry,
using the same address with the following contents:
o Relation Name: alternate
o Description: Designates a substitute for the link's context.
o Reference: [W3C.REC-html401-19991224]
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Internet-Draft HTTP Header Linking March 2008
Language (XLink) Version 1.0", W3C REC REC-xlink-20010627,
June 2001.
Appendix A. Notes on HTML Links
HTML motivated the original syntax of the Link header, and many of
the design decisions in this document are driven by a desire to stay
compatible with these uses.
All of the link relations defined by HTML4 have been included in the
link relation registry, so they can be used without modification.
However, extension link relations work differently in HTML4 and the
Link header; the former uses a document-wide "profile" URI to scope
the relations, while the latter allows the use of full URIs on
individual relations.
Therefore, when using the profile mechanism in HTML4, it is necessary
to map the profiled link relations to URIs. For example, in HTML:
<html>
<head profile="http://example.com/profile1/">
<link rel="foo" href="/foo">
</head>
[...]
could be represented as a header like this;
Link: </foo>; rel="http://example.com/profile1/foo"
Furthermore, the registry defines link relations that are not part of
HTML4. When such relations are used, the absolute URI form should be
used; e.g.,
Link: </head>;
rel="http://www.iana.org/assignments/link-relations.html#current"
HTML defines link relation values as case-insensitive, while the Link
header's syntax does not. Therefore, it is important to case-
normalise relation values in HTML before comparing or converting them
to Link headers.
HTML also defines several attributes on links that are not explicitly
defined by the Link header. Although most of these are believed to
be defunct, they can be used as link-extensions.
Nottingham Expires September 15, 2008 [Page 10]

Internet-Draft HTTP Header Linking March 2008Appendix B. Notes on Atom Links
Atom conveys links in the atom:link element. When serialising an
atom:link into a Link header, it is necessary to convert any IRIs to
URIs, since HTTP headers cannot directly contain UTF-8.
Additionally, since the base URI for link relations in Link headers
is fixed, extension links (i.e,. those not in the registry) MUST be
serialised as absolute URIs.
Note also that while the Link header allows multiple relations to be
associated with a single link, atom:link does not. In this case, a
single Link header may map to several atom:link elements.
As with HTML, atom:link defines some attributes that are not
explicitly mirrored in the Link header syntax, but they may also be
used as link-extensions.
Appendix C. Acknowledgements
This specification lifts the definition of the Link header from
RFC2068; credit for it belongs entirely to the authors of and
contributors to that document. The link relation registrations
themselves are sourced from several documents; see the applicable
references.
Appendix D. Document history
-01
o Changed syntax of link-relation to one or more URI; dropped
Profile.
o Dropped anchor parameter; can still be an extension.
o Removed Link-Template header; can be specified by templates spec
or elsewhere.
o Straw-man for link relation registry.
-00
o Initial draft; normative text lifted from RFC2068.
Nottingham Expires September 15, 2008 [Page 11]

Internet-Draft HTTP Header Linking March 2008
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Nottingham Expires September 15, 2008 [Page 13]